The current radio communication systems are increasingly defined in the form of an allocated frequency band in which communication channels are defined. For a given installation, the choice of the channels used is made as a function of the channels allocated to the operator of the installation and the channels used in the environment of the installation, in order to avoid overlapping phenomena that could lead to interference with the communications (known as cell systems).
For example, the PMR (Private Mobile Radio) and TETRA systems standardized by ETSI (European Telecommunications Standards Institute) use a 5 MHz band, called system band in the remainder of this document, at approximately 400 MHz, and, within this band, the width of each channel is 25 kHz. In such a network, during the installation of a base station, 4 channels will typically be chosen as a function of the criteria mentioned previously in order to cover the communications in the cell centered on the base station, the radius of which corresponds approximately to the range of the system.
A base station thus typically comprises 4 radio receivers, each being dedicated to a given channel.
Each radio receiver of such a base station typically comprises a radiofrequency analog input step connected to an intermediate-band conversion step, the output of which is digitized by an analog-digital converter. The digitized signal is then processed by computers of the signal operation processor type in order to extract the useful information.
The analog input step typically comprises a receiving antenna making it possible to receive the radiofrequency signal. Then this signal is filtered in a band-pass filter called a preselection filter, the bandwidth of which corresponds to the frequency band of the system. The filtered signal is then amplified by a low-noise amplifier before entering the conversion step.
In a mixer connected to a local oscillator, the signal of the selected channel is transposed into a signal at the intermediate frequency, typically of the order of several tens of megahertz.
Conventionally, the transposed signal is then filtered by a band-pass filter having a bandwidth equal to the width of the channel and being centred on the intermediate frequency. At the output of the filter, an analog-digital converter, or ADC, digitizes the signal corresponding to the selected channel before digital processing, the standard configuration of a single-channel digital architecture.
ADCs now exist on the market that have sampling frequencies of approximately 100 MHz and are capable of digitizing to 13 bits ENOB (Effective Number Of Bits). A single ADC then allows the whole of the frequency band of the system to be digitized. Thus, the 4 chains which work independently in parallel on the 4 channels are replaced by a single chain. The transposition stage is followed by a band-pass filter, the bandwidth of which corresponds from then on to the frequency band of the system. This filter is used in order to eliminate the spurious mixing products generated by the transposition. The transposed and filtered signal is then digitized by a single high-frequency ADC. The separation of the channels is then carried out by the downstream digital processing.
This embodiment has the advantage of reducing the number of analog and ADC chains by a factor equal to the number of channels to be digitized.
The counterpart is the greater complication of the digital part which must in particular separate the channels before demodulating each of them.
Moreover, the ADC generates spurious signals which can be impossible to eliminate in the downstream digital steps. The SFDR (Spurious Free Dynamic Range) is the parameter which dimensions the performance of the ADC with respect to this defect.
In a standard fashion, two types of non-linearity give rise to the ADC spurious signals:                The non-linearities of the transfer function of the converter (irregularity of the runs) characterized by INL (Integral Non Linearity) and DNL (Differential Non Linearity) and        The non-linearities of the analog parts of the ADC. These non-linearities generate overtones relating to the signals present at the input of the ADC which fold back and can interfere with the useful signal. Thus in the frequency band of the system, situations can occur in which a useful signal having a relatively low energy is adjacent to the overtone of another signal (useful or interference signal) with relatively high energy. The non-linearity of the ADC can transform this proximity into a noise overlaying the useful signal, noise that is generated by spurious frequencies originating from the interference signal. This phenomenon can be characterized using a spectral analysis at the output of the ADC, the latter being supplied by the interference signal. The analysis then shows a peak at the level of the primary frequency of the interference signal as well as a certain number of spurious peaks, the power of which is potentially greater than the minimum value of a useful signal as defined in a standard. If the frequency of one of these spurious peaks corresponds to the frequency of the useful signal, the latter will experience interference, possibly having a signal/noise ratio that is too low to allow recovery of the information carried.        
In order to reduce the spurious responses due to the transfer function of the ADC, dither noise is commonly used; the addition of a noise uncorrelated with the useful signal still makes it possible to use several ‘runs’ of the ADC, which minimizes the responses linked to the non-linearities of a certain part of the transfer function of the ADC. On the other hand, no effective technique currently exists making it possible to reduce the level of the responses due to the non-linearities of the analog parts.
The signals capable of generating these non-linearities are the set of signals received by the base station, i.e. the useful signals received, those transmitted in the direction of the neighboring base stations and the transmissions of other radiofrequency systems which are not eliminated by the different filters of the receiving chain.
In fact, as previously explained, the channels of the base station are chosen to be different from the channels used by the surrounding base stations in order to avoid interference. However, during the digitization, these frequencies can generate folding overtones, the frequencies of which are in the useful channels and therefore generate a noise that is detrimental to the quality of the transmission.
It would therefore be particularly advantageous to obtain a receiving device that makes it possible to obtain a good signal/noise ratio at the level of the useful channels while minimizing or suppressing the overtones which interfere with these channels.